Kainite mixed salt is obtained from salt bitterns. In pure form it is a double salt of KCl.MgSO4.3H2O although as obtained from salt bitterns it typically contains impurities of NaCl and MgCl2.6H2O.
Potassium sulphate is a dual fertilizer containing 50% K2O and 18% Sulfur. It has other applications also as documented in the prior art.
Ammonium sulphate is used as dual fertilizer containing 21.5% N and 24.6% S. It is also used as raw material for production of various chemicals.
Reference may be made to U.S. Pat. No. 7,041,268, May 9, 2006 by Ghosh et al. which covers extensively the prior art related to production of potassium sulphate (also referred to as sulphate of potash or SOP) from bittern sources. The process co-generates schoenite and KCl which are then reacted together to obtain SOP. Gypsum and magnesium hydroxide/magnesia are obtained as co-products.
Reference may be made to the same patent above wherein the KCl is obtained from carnallite through decomposition followed by a hot leaching process.
Reference may be made to the paper by Amira et al. (Potash Beneficiation “From Hot to Cold Crystallization”, IFA Technical Conference, 1994) wherein it is stated that carnallite may be alternatively subjected to froath floatation to remove excess NaCl and thereafter subjected to decomposition to produce KCl in pure form. The main difficulty with the process is use of organic chemicals required for froth floatation.
Reference may also be made to PCT Publication No WO2010/109492 which teaches the further integration of the above process to obtain ammonium sulphate along with sulphate of potash and magnesium hydroxide/magnesia. The second fertilizer is obtained through the reaction of gypsum with carbon dioxide and ammonia via the Merseberg process to yield ammonium sulphate in solution form and calcium carbonate as precipitate, as taught by the prior art disclosed therein. The calcium carbonate is subsequently re-cycled through its calcinations whereupon the carbon dioxide obtained is utilized in the Merseberg reaction and the lime obtained is utilized in the reaction with magnesium chloride to obtain calcium chloride for desulphatation and magnesium hydroxide as co-product. In this manner, the need for calcium carbonate from external source is dispensed with but the calcium carbonate needs to be of adequate purity to obtain lime of good purity and thus magnesium hydroxide of good purity.
Although the above integration is conceptually ideal, fertilizer companies are typically interested only in fertilizers and may not be interested in the magnesium hydroxide/magnesia component of the product portfolio. Thus a need was felt to devise a process to obtain only sulphate of potash and ammonium sulphate while still ensuring that the process remains viable.
Reference may be made to U.S. Pat. No. 6,776,972 dated Aug. 17, 2004 which discloses the integrated production of pure salt along with marine chemicals. Herein, the decomposition of limestone with hydrochloric acid is disclosed which process generates calcium chloride required for desulphatation operations. No mention is made therein of any utility of the carbon dioxide co-generated.